1. Field of the Invention
This invention relates to neuron stimulation and detection, and, more particularly, to neuromagnetic coil systems and their associated drive circuitry for focused neuron stimulation in selected and electronically controllable subcutaneous regions, as well as for more focused neuromagnetic detection, for example, of biosignals in magnetoencephalography and magnetocardiography.
2. Statement of the Problem
a) Neuromagnetic Stimulation Methods and apparatuses utilizing magnetic stimulation in neural tissues have gained importance for experimentally evoking responses in animals and for some clinical applications. "An Introduction to the Basic Principles of Magnetic Nerve Stimulation" by Barker, "Journal of Clinical Neurophysiology", Vol. 8, No. 1, pp. 26-37, 1991, sets forth the basic principles involving magnetic nerve stimulation. See, also, U.S. Pat. No. 5,047,005 to Cadwell entitled "Method and Apparatus for Magnetically Stimulating Neurons" and U.S. Pat. No. 5,061,234 to Chaney entitled "Magnetic Neural Stimulator for Neurophysiology." As compared to conventional electrical stimulation, such prior magnetic approaches exhibit a number of attractive characteristics. For example, they are non-contacting and noninvasive, they are relatively pain-free, and, most importantly, they can stimulate deeper, normally nonaccessible nerves.
The following references on neuromagnetic stimulation indicate that there is a variety of reports on experimental results utilizing phantoms, animals, and even humans.
In "Developing a More Focal Magnetic Stimulator. Part I: Some Basic Principles", by Cohen and Cuffin, Journal of Clinical Neurophysiology, Vol. 8, No. 1, 1991, pp. 102-111 sets forth a discussion of the magnetic stimulation created by a circular coil parallel to, a circular coil orthogonal to, and a figure-8 coil parallel to the surface of tissue. Cohen et al., recognizes the need to obtain "true focusing" and the lack of any present design to achieve this "true focus." (See page 106). Cohen criticizes the figure-8 coil as being "very inefficient although it exhibited an improvement and focality with decreasing diameter down to 1 cm. In Yunokuchi and Cohen, "Developing A More Focal Magnetic Stimulator. Part II: Fabricating Coils and Measuring Induced Current Distributions", Journal of Clinical Neurophysiology, Vol. 8, No. 1, pp. 112-120, 1991, actual fabricated coils were evaluated.
In "Magnetic Stimulation of the Peripheral Nervous System" by Evans, Journal of Clinical Neurophysiology, Vol. 8, No. 1, 1991, pp. 77-84, the use of a coil in the shape of a butterfly is examined. However, Evans recognizes that when stimulating deeper nerves the patient may experience local pain due to the high current densities involved.
U.S. Pat. No. 5,476,438 entitled "Method and Apparatus for Neuromagnetic Stimulation" and the related article "Neuromagnetic Stimulation Using Ultrasound Focusing: Principles, Limitations and Potential Applications" 27th Annual Meeting Biomedical Engineering Graz., Vol. 38, pp. 415-416, (1993) both by the present inventors set forth an approach using a focused beam of ultrasonic waves interacting into the magnetic field region to produce a subcutaneous focus stimulation having a focal diameter of approximately 1 cm.
Other publications in this field are: S. Ueno et al., "Localized Stimulation of Neural Tissues in the Brain by Means of a Paired Configuration of Time-varying Magnetic Fields", J. Appl. Phys., Vol 64, No. 10, pp. 5862-5864, 1998; Reza Jalinous, "Technical and Practical Aspects of Magnetic Nerve Stimulation", Journal of Clinical Neurophysiology., Vol. 8, No. 1, pp. 10-25 1991; H. Eaton, "Electric Field Induced in a Spherical Volume Conductor from Arbitrary Coils: Application to Magnetic Stimulation and MEG", Med. & Biol. Eng. & Comp., Vol. 30, pp. 433-440, 1992; Anthony Murro et al., "A Model for Focal Magnetic Brain Stimulation", International Journal of Biomed. Comput., Vol. 31, pp. 37-43, 1992; Bradley J. Roth et al., "A model of the Stimulation of a Nerve Fiber by Electromagnetic Induction", IEEE Trans. on Biomed. Eng., Vol. 37, No. 6, 1990; Peter J. Basser et al., "The Activating Function for Magnetic Stimulation Derived from a Three-Dimensional Volume Conductor Model", IEEE Trans. on Biomed. Eng., Vol. 39, No. 11, pp. 1207-1210 1992; Karu P. Esselle et al., "Neural Stimulation with Magnetic Fields: Analysis of Induced Electric Fields", IEEE Trans. on Biomed. Eng., Vol. 39, No. 7, pp. 693-700 July 1992; Karu P. Esselle et al., "Cylindrical Tissue Model for Magnetic Field Stimulation of Neurons: Effects of Coil Geometry", IEEE Trans. on Biomed. Eng., Vol., 42, No. 9, pp. 934-941 September 1995; C. Bischoff et al., "The Value of Magnetic Stimulation in the Diagnosis of Radiculopathies", Muscle Nerve, Vol. 16, pp. 154-161, 1993. The above papers on modeling and simulations of the effects of magnetic stimulation also explore the various mechanisms involved. They show that the induced electric field intensity plays an important role in exciting short nerves, while first order spatial derivatives of electric fields, which are induced along the nerves, act as activating functions towards their excitation. As compared to simple circular coils, the "figure-of-eight" coils (also called "Double-D" or "Butterfly"), the coils with sharp corners, and the "slinky" coils have been reported to yield somewhat improved performance for certain applications.
Yet all of these prior art coil configurations share one main functional disadvantage: they produce maximum field intensity in the surface regions of the tissue including the region that is closest to the coils and considerably lower field intensities in the desired subsurface tissue locations. In other words, they cannot truly focus into deeper subsurface volumes of neuronal tissues as required by most users in animal and general neurophysiological research, as well as in clinical applications. To achieve a desired field intensity at the desired depth, their coils may overheat in high field condition and may overstimulate and overheat superficial tissues. A need exists to overcome these problems and to provide a coil design that provides a peak electric field in a target volume of neuronal tissue thereby increasing focality. A further need exists to control the depth of the peak and to control the orientation of the exciting field vector in the peak.
b) Neuromagnetic Detection--There is a certain similarity, although not simple reciprocity of the stimulation mechanism of nerve tissues by coils, and the inverse, e.g., the detection by coils of the fields naturally emitted by nerve tissues. The latter one is, for example, done in magnetoencephalography (MEG) or in magnetocardiography (MCG) (G. M. Baule, N.Y. State J. Med., 67, p. 3095, 1967; J. E. Zimmerman, J. Appl. Physics 48, p. 702, 1977; M. Reite, J. E. Zimmerman, J. Edrich, H. Zimmerman, Electroenc. Clin. Neurophysiol. 40. p. 59, 1976; W. J. Williamson, et al., J. Magnetism and Magn. Mat. 22, p. 129, 1981; S. N. Erne et al., ed., Biomagnetism, W. D. Gruyter, Berlin, 1981; P. Weismuller, J. Edrich et al., PACE Vol. 14 p. 1961, 1991; R. Kristeva-Feige, S. N. Erne, J. Edrich, et al., Abstr. BIOMAG 96, p. 268, 1996.
Therefore it can be expected that some, although not all, of the above mentioned problems associated with stimulation also occur with biomagnetic detection. In fact, the problems due to limited spatial localization accuracy of detecting current sources associated with neural activities in the brain and with the electrophysiological activities of the heart are presently of acute concern. (Abstr./Proc. BIOMAG 96; Santa Fe, pp. 1-340, 1996). Many of the industrial and development groups optimizing MEG and MCG (for functional imaging in neurophysiological or clinical application) are presently trying to overcome the localization problems of conventional biomagnetic coil systems by using so-called multichannel systems. These approaches also use many coils placed side-by-side over the nerve tissue to be measured. For example 150 coils or coil systems, are used in helmet like structures for the brain (MEG) or 55 coils or coil systems are used in planar, circular arrays for the heart (MCG). In contrast to stimulation with fields in the Tesla range, here the fields produced by the nerve currents are of the order of 10.sup.-10 Tesla or less, i.e., very weak, although still accurately measurable using conventional superconducting coils and Josephson detectors. Depth localization or subcutaneous current sources or dipoles are facilitated and made more accurate by using the above multi-coil/multichannel approaches. However, the accuracy is still very marginal for many neurophysiological and clinical applications, in particular, if comparison with structural imaging methods, such as MRI and CAT, is needed, as is required by many users. Focusing into deeper tissue regions is, for example, especially important for the relatively deep current sources of the endocard using MCG. Here and also in MEG applications (preoperative localizations of epileptic foci, schizophrenia etc.), the so-called "biological noise", is also a major source of problems. It is caused by the pickup and detection of unwanted fields due to nerve currents in volumes that are not of interest, but are, unfortunately, included because of the coarse volume resolution of conventional coils. Similar to the stimulating coils, here the inductance plays a major role in the signal-sensitivity of coils (J. E. Zimmerman, J. Appl. Phys. 48, p. 703, 1977). All of the above problems point at the need for improved coils or coils system with significantly higher spatial resolution at subcutaneous depths that are of interest for most neurophysiological and clinical applications.
A need exists for a coil system that focuses on a target volume or region of neuronal subcutaneous tissue. A need also exists to provide electronic weighting of biomagnetically detected coil currents and the determination of the direction of vectorial nerve current dipoles.
In one application, the coil design stimulates the target volume of neuronal tissue with a peak of electric field energy. In a second application, the coil design detects in the target volume of neuronal tissue electrical fields generated by nerve currents.
3. Solution to the Problem
It is an objective of this invention to provide an effective method and apparatus for focusing on a target region of subcutaneous nerve tissue for neuromagnetic stimulation or for biomagnetic detection of naturally occurring nerve currents.
It is a further objective of the invention to provide a method and apparatus to avoid the focusing problems and to reduce overheating of coils of conventional magnetic stimulators under high field conditions and to significantly reduce the problems in biomagnetic detection that are caused by noise and coarse spatial resolution.
It is a still further objective of the invention to provide a method and apparatus for neuromagnetic stimulation that provide a more focused stimulation of selected subcutaneous nerve tissue regions without over stimulating and overheating superficial tissues and, similarly, provide a more focused low noise detection of deeper biomagnetic signals.
It is still a further objective of the present invention to provide a method and apparatus for neuromagnetic stimulation that controls the depth and the orientation of the exciting field vectors and for biomagnetic detection that electronically weighs the detected currents and determines the direction of vectorial nerve current dipoles.